Alkali metal silicate binder for foundry sand molds and process



United States Patent ALKALI METAL SILICATE BINDER FOR FOUNDRY SAND MOLDS AND PROCESS Frank P. Ilenda and Cletus E. Peeler, Jr., Painesville,

Ohio, assignors to Diamond Alkali Company, Cleveland, Ohio, a corporation of Delaware No Drawing. Application February 29, 1956 Serial No. 568,442

7 Claims. (Cl. 106-3835) This invention relates to improvements in alkali metal silicate-containing mold-forming compositions adapted to be cured or hardened by impregnation with carbon dioxide or other gaseous curing agent, and to methods for their preparation and use.

In recent years there has been considerable interest in the preparation of molds and cores from sand-silicate mix tures which are cured or hardened in the desired castingdefining shapes by injections of carbon dioxide or other gaseous materials. Generally, the curing of molds embodying binders of this type involves a chemical reaction between carbon dioxide or other gaseous material and the alkali metal silicate dispersed throughout the sand or other refractory material to form a uniformly-dispersed silicon dioxide gel binder.

The advantages of such molds are many. Not only are aging and post-curing treatments eliminated, thus permitting rapid fabrication of molds without requiring the use of expensive equipment, but also close tolerance castings are obtainable and the process is relatively simple and can be carried out readily on a large scale.

However, despite the many advantages of such molds, it frequently has been found that certain difficulties are encountered with prior silicate-containing mold-forming compositions of this type, especially in the casting of high temperature melting metals, such as cast iron and steels melting at temperatures of 2500 F. or higher.

At high casting temperatures, prior silicate-containing mold-forming compositions generally have been characterized by excessive evolution of steam and/or other gases during casting. While mold porosity certainly is essential, often when sufficient porosity is provided to permit escape of the large volume of gases generated using prior mold-forming compositions in high temperature casting of cast iron and steels, such molds have not achieved the widespread application they might otherwise enjoy.

Accordingly, it is the principal object of the present invention to avoid the difiiculties heretofore encountered and to provide alkali metal silicate-containing compositions adapted for use in forming improved molds for metal casting operations.

A further object of the invention is to provide new and improved compositions for forming molds adapted to be cured by exposure to carbon dioxide or other gaseous materials.

A still further object of the invention is to provide new and improved methods of metal casting.

A still further object of the invention is to provide an improved shell mold and methods of shell molding.

These and other objects and advantages of the invention will appear more fully from the following description thereof.

As used throughout the specification and claims, the term mold is intended in a generic sense to mean cast ing forms which include both molds and cores, this invention in no manner being limited to the former. Moreover, mold is intended to include various patterns for 'mally solid at room temperature.

use in the casting art, as well as shell molds including shell mold-forming elements in addition to the completed shell mold structure prepared by assembling two or more complementary thin-walled shell mold elements.

This invention is an improvement over the invention described and claimed in co-pending application Ser. No. 549,850, filed November 29, 1955, and Ser. No. 540,633, filed October 14, 1955. While the inventions there described provide greatly improved results over the prior art, it has now been found that in many instances even more advantageous results are obtained with the practice of the present invention, especially with respect to casting of high melting metals.

Accordingly, molds formed in accordance with the present invention are especially advantageously employed in the casting of metals melting at a high temperature, e.g., cast iron and/or various steels.

The present invention contemplates a binder for finelydivided refractory material comprising the composition obtained by combining a liquid alkali metal silicate and a solid alkali metal silicate. Another embodiment of the invention contemplates a binder composition obtained by combining a liquid alkali metal silicate, a finely-divided solid, soluble, alkali metal silicate and an organic compound.

The foregoing compositions are particularly advantageous when employed as binders for sand or other finelydivided refractory materials employed in preparation of metal casting molds cured by exposure to carbon dioxide or other gaseous material. Hence, it will be appreciated that the practice of this invention preferably contemplates composition of matter comprising a major proportion of at least one finely-divided refractory material, preferably sand, and a minor proportion of a binder comprising the material obtained by combining a liquid alkali metal silicate and a finely-divided solid, soluble alkali metal silicate.

Another embodiment of the invention comprises a mixture of a major proportion of a finely-divided refractory material, preferably sand, and a minor proportion of a binder comprising the material obtained by combining a liquid alkali metal silicate, a finely-divided solid, soluble alkali metal silicate and an organic compound.

More specifically, the present invention contemplates the use in the preparation of gas-curable molds of a binder, a liquid alkali metal silicate, i.e., an aqueous solution of a silicate normally liquid at room temperature, having a silica to alkali metal oxide molar ratio greater than about l.0:1.0, preferably within the range from about 1.5 to 3.3, i.e., an alkali metal oxidezsilicia ratio of from about 10:15 to 1.0133, such as an Na O:SiO ratio of 1.0: 1.5 to 1.0:3.3, said silicate having added thereto a solid, soluble, alkali metal silicate, i.e., a silicate nor- The solid silicate generally may vary in alkali metal oxide to silica molar ratio from about 2.0:l.0 to 1.0133, such as an Na OzSio ratio of about 2.0:l.0 to l.0:3.3, soluble, solid sodium silicates having an Na O:SiO molar ratio of about 1.0:].0 being preferred, such as sodium metasilicate. As indicated above, silicates having silicazalkali metal oxide molar ratios greater than about 1.0:l.0 are generally given in terms of weight ratios, while silica to alkali metal oxide ratios of silicates having a silica:alkali metal oxide molar ratio of 1.0: 1.0 or less are generally given as molar ratios. In a preferred embodiment, soluble, solid sodium silicates are employed, notably anhydrous sodium metasilicate. The particle size of the soluble, solid alkali metal silicate generally can vary somewhat, the upper limit on particle size being dictated by the desired rate of solution. However, in most instances superior results are obtained using particles less than 60 US. mesh size, a preferred particle size range being from 60 to +200 U.S. mesh size.

While the proportions of components included in compositions of this invention may, of course, be varied somewhat, it generally is preferred to employ a major proportion of a finely-divided refractory material and a minor proportion of a silicate binder. In this connection, it generally is preferable to employ mixtures of finely-divided refractory materials and silicate binders of this invention wherein the refractory material comprises from about 90 to 97% by weight of the mold-forming composition as employed, a preferred range being from about 90 to 95% by weight, and the silicate binder constituting the balance. Considering the silicate-containing binder compositions per se, it has been found that superior results are achieved when the liquid alkali metal silicate comprises about 80 to 99% by weight of the total silicate-containing binder, the soluble, solid alkali metal silicate comprising about 1 to 20%, the maximum upper limit being dictated by its solubility.

In many instances, in order to improve the collapsibility of fired mold compositions and to improve each life ngl kfpzmmn c os'itions, it is desirable'tofiric liide in the mold-forming composition an organic compound which is burned out during the casting operation and thus to promote collapsibility of the fired mold. In the practice of this invention, organic compounds, preferably polyhydroxy organic compounds may be incorporated in minor amounts in the silicate-containing mold-forming composition. The expression polyhydroxy organic compound as used in the specificathrand cl'air'ns is intended'to include a variety of organic compounds containing more than one hydroxy grouping. While the especially preferred compounds are polyhydroxy in nature, it is intended to include various sue rs such as glucose, fructose, sucrose, and various otfi fionoand disaccharides as well as certain triand tetrasaccharides, which compounds, of course, need not be in a highly purified state, excellent results having been obtained using relatively impure sugar, such, e.g., as tanners sugar, blackstrap molasses, bagasse, as well as other low cost sugar products. In addition, another type of organic polyhydroxy compound suitable for use in the practice of this invention is a polyhydroxy alcohol such as glycerol, sorbitol, and other hydroxy alcohols. Presently preferred organic compounds are sorbitol, glycerine and sugars, notably sucrose.

In those applications where an organic compound is employed, it has been found that excellent results are obtained using a mixture containing about 60 to 95% by weight liquid alkali metal silicate, 1 to 20% by weight soluble, solid, alkali metal silicate and about I to 2.1% by Wpound, preferably a polyhydroxy organic compound. n ferrous metallurgy involving casting at temperatures of 2500* to 3000 F., it is preferred to employ a composition comprising about 90% by weight of a liquid sodium silicate having a silica to alkali metal oxide weight ratio within the range from about 2.0 to 3.3, preferably about 2.84, about 5% by weight anhydrous sodium metasilicate having a particle size within the range from 60 to 200 mesh, and about 5% by weight of sorbitol, glycerine, or a mixture thereof.

In the casting of non-ferrous materials at temperatures generally within the range from l400 to 2000 F., a presently preferred silicate binder composition comprises about 85% by weight of a liquid sodium silicate having a silica to alkali metal oxide weight ratio within the range from about 2.0 to 3.22, a 3.22 ratio material being preferred at the present, about 5% by weight anhydrous sodium metasilicate having a particle size within the range from --60 to +200 mesh and about by weight of organic compounds, notably sucrose in the form of crude or refined cane or beet sugar.

When no organic compound is employed, silicate composition embodying the present invention generally comprise about to 99% liquid alkali metal silicate and 1 to 20% soluble solid alkali metal silicate. Illustrative of a typical preferred formulation of this type for use in ferrous metallurgy, i.e., when casting metals melting at temperatures within the range from about 2500 to 3000 F. or higher is a composition comprising about 10% by weight of at least one soluble solid alkali metal silicatc. notably anhydrous sodium metasilicate having a particle size between about 60 and +200 mesh, and about by weight of at least one liquid sodium silicate having a silica to alkali metal oxide ratio within the range from about 2.0 to 3.3.

When lower casting temperatures are to be employed. e.g., in non-ferrous metallurgy such as the casting of bronze and aluminum at temperatures within the range from about l400 to 2000 F., a preferred binder composition consists essentially of 5% by weight of at least one soluble solid alkali metal silicate. notably anhydrous sodium metasilicate having a particle size within the range from 60 to 200 mesh and about Q of at least one liquid sodium silicate having a silica to alkali metal oxide ratio within the range from about 2.0 to 3.33.

The expression alkali metal" as used throughout the specification and claims is intended to refer to the various alkali metals, i.e., sodium, potassium, rubidium. cesium and lithium, sodium generally being preferred.

While carbon dioxide or gases containing carbon dioxide comprise a presently preferred form of curing agent for refractory material-silicate binder mold-forming compositions of this invention, it is to be understood that other gaseous materials or mixtures of gases also may be employed. For example, sulphur dioxide, nitrous oxide, hydrogen chloride, and the like, also may be used. Moreover, in many instances, various combustion or stack gases containing proportions of carbon dioxide and the like also may be utilized advantageously. The length exposure to the curing gas required to effect the desired cure depends, of course, on a number of variables including mold mass porosity, gas employed, and the like. However, the time required typically is of the order of a few seconds, e.g., 5 to 10 to several minutes, e.g., 10 minutes.

Whilesand constitutes a preferred refractory material because of its widespread foundry usage, low cost and availability, other refractory materials also may be used advantageously such, e.g., as silica flour, and various other inorganic refractory materials.

The high solids content provided by the practice of this invention insures the presence of sufiicient silica to provide an excellent inorganic binder, i.e., silica in the form of a gel uniformly dispersed throughout the mold, in order to provide and maintain the necessary mold strength during metal solidification in casting whereby mold erosion is minimized and excellent cast metal tolerances are achieved.

In order that those skilled in the art may more completely understand the present invention and the preferred methods by which the same may be carried into effect, the following specific examples are offered:

Example I To illustrate the practice of the present invention, a silicate binder composition consisting of 90% by weight liquid sodium silicate (lNa O:2.84SiO containing 43.1% solids, and 10% by weight finely-divided anhydrous sodium metasilicate (-60 to +200 mesh) is prepared by adding the solid silicate to the liquid silicate. Using the thus-prepared binder (solids content 48.8%) a sand-silicate mixture is prepared by admixing 95% by weight of 80 grain fineness sand, and 5% by weight silicate binder. This mixture is formed into 2 inch square cubes which are then impregnated with carbon dioxide for 10 seconds. The thus-treated cubes have an unfired or green strength of 277 psi.

Example II The procedure according to Example I is repeated using 90% by weight liquid sodium silicate (1Na- O:2.4SiO containing 46.6% solids, and by weight powdered anhydrous sodium metasilicate. The resultant silicate mixture has an Na O:SiO ratio of 1:2.0 and a solids content of 51.94. Compressive strength of a two inch square cube of the thus-formed 95% sand5% silicate composition after exposure to carbon dioxide for 10 seconds is 172 p.s.i. A similarly-proportioned sand-silicate mixture employing a silicate binder consisting of 100% liquid sodium silicate (1Na O:2.0SiO containing 43.5% solids has a compressive strength of but 55 p.s.i.

Example III Using the procedure of Example I, a 95% sand-5% silicate composition is prepared, the silicate comprising a mixture of 93% by weight liquid sodium silicate (lNa O:3.22SiO containing 38.35% solids, and 7% by weight powdered anhydrous sodium metasilicate. The resulting silicate mixture (1Na O:2.59SiO has a solids content of 42.65%. The compressive strength of a two inch square cube of such a material after impregnation with carbon dioxide for 10 seconds is 177 p.s.i.

Example IV The process of Example I is repeated using as a binder a composition obtained by mixing 95% by weight liquid sodium silicate (1Na O:3.22SiO and 5% by weight powdered anhydrous sodium metasilicate, the resulting silicate mixture (1Na O:2.75SiO having a solids content of 41.45%. The compressive strength of a two inch square cube formed of such a sand-silicate mixture after impregnation with carbon dioxide for 10 seconds is 167 p.s.i. A comparative test using as the silicate a liquid sodium silicate (1Na O:3.3SiO having a solids content of 37.3% indicates a compressive strength of only 67 Example V Using the procedure of Example I, a sand-silicate composition is preparcd containing 5% of a silicate binder which consists of 88.35% by weight liquid sodium silicate (1Na O:3.22SiO containing 38.55% solids, 4.65% anhydrous sodium metasilicate, and 7.00% glycerine, the resultant silicate mixture having the ratio The compressive strength of such a sand-silicate mixture after impregnation with CO for 10 seconds is 168 p.s.i.

Example VI Using the procedure of Example I, a sand-silicate composition is prepared containing 5% of a silicate binder which consists of a mixture of 90% by weight liquid sodium silicate (lNa O:2.4SiO containing 46.6% solids, 5% by weight anhydrous sodium metasilicate, and 5% by weight sucrose, the resultant silicate mixture having the ratio Na O:2.03SiO The compressive strength of such a sand-silicate mixture after impregnation with CO for 10 seconds is 372 p.s.i.

Example VII The procedure of Example I is repeated using a 5% silicate binder comprising a mixture of 82% by weight liquid sodium silicate (1Na O:3.22SiO containing 38.55 solids, 7% by weight anhydrous sodium metasilicate, 10% by weight sucrose, and 1% by weight parafiin oil, the resulting silicate mixture having the ratio 1Na O:2.52SiO The compressive strength of such a sand-silicate mixture after impregnation with CO for 10 seconds is 162 p.s.i.

Example VIII The procedure of Example I is repeated using as the silicate binder 5% of a mixture consisting of 90% by weight liquid sodium silicate (lNa O:2.84SiO containing 43.1% solids, 5% by Weight anhydrous sodium metasilicate, and 5% by weight sucrose, the resultant silicate mixture having the ratio 1Na O:2.45SiO The compressive strength of such a sand-silicate composition after impregnation with CO for 10 seconds is 333 p.s.i. A similar comparative test using 5% of a mixture of 95 liquid sodium silicate (1Na O:2.4SiO and 5% sorbitol indicates a compressive strength of but 107 p.s.i.

Example IX One ton of a sand-silicate binder composition embodying the invention is prepared by mixing mesh sand with 5% by weight of silicate binder consisting of liquid sodium silicate (1Na O:2.84SiO and 10% by weight anhydrous sodium metasilicate.

The thus-prepared sand-silicate mixture is pressed against the face of a pattern with an air hammer to a thickness of 4 to 5 inches and backed with green sand. The thus-prepared material is impregnated with carbon dioxide for seven minutes. After the pattern is withdrawn, the casting-defining surface is brushed with a conventional alcohol-graphite wash. The mold is then assembled and cast. Following solidification, an excellent casting is obtained with substantially no penetration or veining noted.

The following silicate compositions illustrate use of solid silicates other than sodium metasilicate in composition embodying the invention.

Example X 83% liquid sodium silicate (1Na O:2.84SiO 17% powdered sodium disilicate (1Na O:2SiO

(17% B 0) Resultant mixture1Na O:2.56SiO total solids 49.0%

Example XI liquid sodium silicate (1Na O:2.4SiO 5% solid sodium silicate (1Na O:3.22SiO Resultant mixture-1Na O:2.46SiO -total solids 48.5%

Example XII It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. A metal casting mold adapted for use in casting of high melting metals and alloys, said mold being formed from a mixture of a major proportion of about 90% to 97% by weight of a finely-divided refractory and a minor proportion of about 3% to 10% by weight of a liquid binder comprising in combination about 1 to 20%, inclusive, of a finely-divided water-soluble solid alkali metal silicate having an alkali metal oxidezsilica ratio within the range of 2.0:1.0 to 1.0:3.3, inclusive, and about 80% to 99% of a liquid alkali metal silicate having a solids content of about 38% to 46% by weight, the resultant mold when impregnated with carbon dioxide having an unfired strength substantially greater than that obtained using a liquid alkali metal silicate of the same total solids content employed without addition of the said solid alkali metal silicate.

2. A mold as in claim 1 wherein there is also included in the binder about 1% to 25% of the total binder weight of a polyhydroxy organic additive.

3. A composition of matter comprising the material obtained by combining about 60% to 95% by weight of a liquid aqueous alkali metal silicate having a silicazalkali metal oxide ratio within the range from 1.5 to 3.3:1.0 and a solids content of about 38% to 46% by weight, about 1% to 20% by weight of a finely-divided soluble solid alkali metal silicate having an alkali metal oxide:sil ica ratio within the range of 2.0:l.0 to l.0:3.3, inclusive, to increase the total solids content of the resultant liquid composition and about 1% to 25% by weight of a polyhydroxy organic compound.

4. A composition of matter consisting essentially of a major proportion of finely-divided refractory material and a minor proportion of the composition of claim 3.

5. A composition of matter adapted for use in forming metal casting molds consisting essentially of, in combination, an intimate mixture of about 90% to 97% of a finely-divided refractory material and the balance a binder consisting essentially of about 60% to 95% by weight of a liquid sodium silicate having a SiO :Na O ratio within the range from about 1.5 to 33:10, inclusive, and a solids content of about 38% to 46% by weight, to which has been added about 1% to 25% by weight of a polyhydroxy organic compound and about 1% to 20% by weight of finely-divided, solid sodium metasilicate.

6. A composition of matter comprising the mixture obtained by combining about 60% to 95% by weight of a liquid alkali metal silicate having a silica to alkali metal oxide ratio within the range from about 1.5 and 33:10, inclusive, and a solids content of about 38% to 46% by weight, and about 1% to 20% by weight of a finelydivided soluble, solid alkali metal silicate having a silica to alkali metal oxide ratio within the range from about 2.0 alkali metal oxide:l.0 silica to 1.0 alkali metal oxide:3.33 silica, inclusive, the resultant composition when impregnated with carbon dioxide having an unfired strength substantially greater than that obtained using a liquid alkali metal silicate of the final total solids content employed without addition of the said solid alkali metal silicate.

7. A mold according to claim 1 wherein the resultant mold is a shell mold.

References Cited in the file of this patent UNITED STATES PATENTS 2.368.322 Passelecq Ian. 30, 1945 2,480,896 Bean Sept. 6, 19-19 2,494,403 Nies et al. Jan. 10, 1950 2,521,839 Feagin et a1 Sept. 12, 1950 2,732,600 Hanink et al. Jan. 31, 1956 2,748,435 l-lackett June 5, 1956 2,749,586 Kohn et a1. June 12, 1956 2,787,968 Luvisi Apr. 9, 1957 2,861,893 Brewster Nov. 25, 1958 FOREIGN PATENTS 710,099 Great Britain June 9, 1954 745,402 Great Britain Feb. 22, 1956 

1. A METAL CASTING MOLD ADAPTED FOR USE IN CASTING OF HIGH MELTING METALS AND ALLOYS, SAID MOLD BEING FORMED FROM A MIXTURE OF A MAJOR PROPORTION OF ABOUT 90% TO 97% BY WEIGHT OF A FINELY-DIVIDED REFRACTORY AND A MINOR PROPORTION OF ABOUT 3% TO 10% BY WEIGHT OF A LIQUID BINDER COMPRISING IN COMBINATION ABOUT 1 TO 20%, INCLUSIVE, OF A FINELY-DIVIDED WATER-SOLUBLE SOLID ALKALI METAL SILICATE HAVING A ALKALI METAL OXIDE: SILICA RATIO WITHIN THE RANGE OF 2.0:1.0 TO 1.0:3.3, INCLUSIVE, AND ABOUT 80% TO 99% OF A LIQUID ALKALI METAL SILICATE HAVING A SOLIDS CONTENT OF ABOUT 38% TO 46% BY WEIGHT, THE RESULTANT MOLD WHEN IMPREGNATED WITH CARBON, DIOXIDE HAVING AN UNFIRED STRENGTH SUBSTANTIALLY GREATER THAN THAT OBTAINED USING A LIQUID ALKALI METAL SILICATE OF THE SAME TOTAL SOLIDS CONTENT EMPLOYED WITHOUT ADDITION OF THE SAID SOLID ALKALI METAL SILICATE. 